Conventionally this type of positioner is designed so as to operate with an electric current between 4 and 20 mA (a DC electric signal) sent through a pair of electric wires from a higher-level system. For example, with the regulator valve as a proportional valve if a current of 4 mA is sent from the higher-level system, the opening of the proportional valve is set to 0%, and if a current of 20 mA is sent, then the opening of the proportional valve is set to 100%.
In this case, the supplied electric current from the higher-level system varies in the range of 4 mA (the lower limit electric current value) through 20 mA (the higher limit electric current value), and thus the internal circuitry within the positioner produces an operating power supply itself from an electric current of no more than the 4 mA that can always be secured as an electric current value that is supplied from the higher-level system (See, For Example, Japanese Unexamined Patent Application Publication H1-141202 (“JP '202”)).
The opening setting value for the regulator valve is inputted into the positioner by the higher-level system. Moreover, the actual opening value for the regulator valve is obtained through the opening sensor. Consequently, the positioner is able to perform regulator valve fault diagnostics, self-diagnostics, and the like, through performing calculations on the relationship between the opening setting value and the actual opening value for the regulator valve, The provision of such fault diagnostic functions in the positioner makes it possible to increase the functionality of the system at a low cost, through eliminating the need for providing a separate fault diagnosing device (See, for example, JP '202).
For reasons such as these, in recent years there have been proposals for positioners that have, in addition to their actual functions of controlling the degree of opening of the regulator valves, also opening degree transmitting functions, regulator valve fault diagnostics, and functions for sending, to the higher-level system, the results of fault self-diagnostics, and the like. FIG. 8 shows the structure of the critical components in a system that uses a positioner that has a communication function for the higher-level system.
In FIG. 8: 1 is a positioner; 2 is a higher-level device that is connected to the positioner through double-wire transmission lines (a pair of electric wires) L1 and L2; 3 is a communication device that is connected, as necessary, between the transmission lines L1 and L2; 4 is a higher-level system; and 5 is a regulator valve (proportional valve). The positioner 1 is provided with a main circuit 1-1, a communication circuit 1-2, and a constant voltage circuit 1-3. Note that in the present example, the higher-level system 4 is structured from the higher-level device 2 and the communication device 3.
In this system, the higher-level device 2 sends a 4 to 20 mA DC electric current signal I through the transmission lines L1 and L2 to the positioner 1. In the positioner 1, the constant voltage circuit 1-3 generates a constant voltage Pvos from the DC electric current signal I that is sent from the higher-level device 2, and supplies it to the communication circuit 1-2 and the main circuit 1-1. The main circuit 1-1 controls the opening of the proportional valve 5 depending on the value of the DC electric current signal I that is sent from the higher-level device 2. Moreover, it also performs fault diagnostics on the proportional valve 5, fault self-diagnostics, and the like.
The communication device 3 superimposes an AC electric current signal If for communication on to the DC electric current signal I to the positioner 1. In the positioner 1, the communication circuit 1-2 is an electric current inputting-type communication circuit, and extracts the AC electric current signal If that is superimposed on the DC electric current signal I, and sends, to the main circuit 1-1, instructions and data from the communication device 3, sent via the AC electric current signal If. Moreover, the communication circuit 1-2 sends, to the communication device 3, the results of the fault diagnostics on the proportional valve 5 and the results of the fault self-diagnostics from the main circuit 1-1 through changing the voltage between the transmission lines L1 and L2. This type of communication method is disclosed in, for example, Japanese Unexamined Patent Application Publication S61-070827 (“JP '827”).
Note that recently there have been requests desiring that fault diagnostics be performed not just on proportional valves, but on ON/OFF valves as well, and there are cases wherein positioners are employed. However, ON/OFF valves use two-level control (all the way open or all the way closed), and so normally an electromagnetic valve is used, and normally a DC voltage signal (normally between 0 and 24 V) is inputted from the pair of electric wires in order to actuate that electromagnetic valve. Given this, it is necessary for the internal circuitry within the positioner that controls the valve opening of the ON/OFF valve to be compatible with this voltage input.
For example, in JP '827, if one of the communicating devices is a positioner and the other communicating device is the higher-level device, then an AC voltage signal for communication is superimposed on a DC voltage signal, that varies over a specific range, and sent from the higher-level device to the positioner side, and, on the positioner side, the DC voltage signal is extracted from the voltage signal that has been received from the higher-level device, and the degree of opening of the regulator valve is controlled based on the value of this DC voltage signal, while, at the same time, the electric current of the line that returns to the higher-level device is varied, making it possible to communicate the degree of valve opening and the fault diagnostics results to the higher-level device side. In this case, the positioner is voltage-input compatible, and can be applied to an ON/OFF valve. Moreover, the power supply of the higher-level system may be of a voltage-outputting type (DO: Digital Output) instead of the current-outputting type (AO: Analog Output), and may also control the opening of a proportional valve.
Note that the voltage output-type system for supplying power is known as a field bus system (referencing, for example, Japanese Unexamined Patent Application Publication 2004-226092 (Japanese Patent Number 4185369)), where a voltage of for example, between 9 and 32 V is supplied by the higher-level system.
Moreover, in the below, a positioner of the type that inputs a DC electric signal shall be termed an electric current inputting-type positioner, and a positioner of the type that inputs a DC voltage signal shall be termed a voltage inputting-type positioner.
In this case, the manufacturer that manufactures and supplies the positioner must prepare two models of positioners, the electric current inputting-type positioner and the voltage inputting-type positioner, in order to be compatible with ON/OFF valves and the compatible with those of the field device-type, while providing a function for communicating with the higher-level system, which increases the manufacturing overhead. Moreover, the voltage inputting-type positioner is expensive because it must be prepared as a type that is different from the typical electric current inputting-type positioner.
Given this, in order to eliminate this overhead, Japanese Unexamined Patent Application Publication 2002-367069 (“JP '069”) discloses a positioner of a joint-use type wherein a single model is compatible both with the electric current input from an analog transmission line and a voltage input from a field device transmission line. This joint-use positioner is provided with an interface circuit (I/V block) for connecting to an analog transmission line and an interface circuit (FB block) for connecting with a field device transmission line, and is switched as necessary between the I/V block and the FB block.
However, in the joint-use positioner disclosed in JP '069, the I/V block and the FB block must be provided separately, causing the structure to be complex, and producing a problem wherein it is more expensive than the electric current inputting-type positioner and the voltage inputting-type positioner.
Given this, one of skill in the art can conceive of providing a fixed resistor 1-4 (referencing FIG. 9) in a stage prior to the communication circuit 1-2 in the electric current inputting-type positioner 1 (1A) illustrated in FIG. 8. Note that in a system that uses this positioner 1 (1B), the higher-level device 2 (2B) sends a DC voltage signal V to the positioner 1B. Moreover, the communication device 3 (3B) superimposes an AC voltage signal Vf for communication onto the DC voltage signal V to the positioner 1B. Moreover, the regulator valve 5 (5B) is an electromagnetic valve (ON/OFF valve).
In this positioner 1B, the DC voltage signal V from the higher-level device 213 is converted into an electric current by the fixed resistor 1-4 of a resistance value r, where this electric current is sent to the main circuit 1-1. Through this, the main circuit 1-1 controls the opening/closing of the regulator valve 5b based on the value of the electric current that was converted by the fixed resistor 1-4, that is, based on the value of the DC electric current signal V/r. Note that a load resistance 6 that is larger than the resistance value r of the fixed resistor 1-4 is provided in the line L1 to prevent the incursion of the communication signal into the voltage source side. Moreover, the resistance value r of the fixed resistor 1-4 is made small, and the AC voltage signal Vf is converted into a change in the electric current.
However, if this type of positioner 1B were connected in error to a high-voltage power supply without connecting the load resistance 6 (referencing FIG. 10), then, because the resistance value r of the fixed resistor 1-4 is small, an excessively large electric current will flow into the internal circuitry of the positioner 1B, risking damage to the positioner 1B.
Given this, as illustrated in FIG. 11, one may consider having a resistance value r that is larger than that of the resistance value of the fixed resistor 1-4, but when this is done, the impedance relating to the AC voltages signal Vf becomes too high relative to the power supply side, so that the converted electric current becomes small, making communication impossible.
The present invention is to solve such problems, and the object thereof is to provide a voltage inputting-type positioner of an inexpensive structure wherein, through a simple change relative to the common electric current inputting-type positioner, there will be no excessively large electric current even if connected to the voltage supply side without connecting a load resistance, so that communication will also be possible without impediment.